动态生成DropDownList控件后，在选择改变时获得DropDownList.SelectedItem.Text的值

[永远爱好写程序]

代码如下,请问如何实现在DropDownList控件选择改变时(SelectedIndexChanged事件)
让ViewState["UpdateDepartmentName"])得到当前选择的文本??
要求以上功能在如下的DataGrid1_ItemDataBound事件中实现

private void DataGrid1_ItemDataBound(object sender, System.Web.UI.WebControls.DataGridItemEventArgs e)
{
if(e.Item.ItemType==ListItemType.EditItem)
{
DropDownList ddl=new DropDownList();
DataSet objDs=new DataSet();
objDs=clsBase.ExecuteSQL("Select DepartmentName from tblDepartment");
ddl.DataSource=objDs.Tables[0].DefaultView;
ddl.DataTextField="DepartmentName";
ddl.DataValueField="DepartmentName";
ddl.DataBind();
if(e.Item.Cells[5].Controls[0].ToString()=="System.Web.UI.WebControls.TextBox")
{
TextBox txtBox=(TextBox)e.Item.Cells[5].Controls[0];
ddl.SelectedIndex=0;
while(e.Item.Cells[5].Controls.Count>0)
{
e.Item.Cells[5].Controls.RemoveAt(0);
}
e.Item.Cells[5].Controls.Add(ddl);
ViewState["UpdateDepartmentName"]=ddl.SelectedItem.Text;
}
}
}